The present invention is related to a compact automatic gear change device for a bicycle in which a first set of sprocket wheels (1) mesh with a second set of sprocket wheels (2) by means of an intermediate moving element (3), and are equipped with anti-reverse pawls automatically controlled by mechanical or electronic tachometers.
French Patent No. 2.711.351, published on Apr. 28, 1995, describes a differential selection gear change for a bicycle, based on the difference in the angular speed of the pinions, and on the utilization of anti-reverse pawls, each controlled pawl having a lug added to enable their deactivation to be controlled. The sprocket wheels are free on their axles and mesh constantly by means of an intermediate moving element. Due to the anti-reverse pawls, the sprocket wheels only rotate in one direction and cause the engagement of the highest ratios in a natural fashion by the difference in the angular speed of the sprocket wheels. The speed selection is done by lifting the pawls by means of their lug. Since very little force is necessary to lift a pawl, this device is particularly adapted to the embodiment of a fully automatic gearbox for a bicycle due to mechanical or electronic tachometers measuring the speed of the moving elements and the transmitted torque.